Membrane lipid degradation is related to interictal cortical activity in a series of seizures

Brain levels of free fatty acids (FFA) and diacylglycerols (DAG) rise rapidly with the onset of seizures, reflecting activation of phospholipases A2 (PLA2) and C (PLC), respectively. However, the ictal/interictal accumulation of FFA attenuates as recurrent seizures continue. To assess the role of neuronal activity in stimulating PLA2 and C, we compared FFA and DAG in rat cerebral cortex during recurrent ictal periods as a function of associated levels of interictal activity. Pentobarbital-anesthetized rats were paralyzed, ventilated with 30% O2 and subjected to periodic pentylenetetrazol seizures at intervals of 5 min. Animals were killed with focused-microwave irradiation during either the 3rd or 15th seizure. The rise in cortical FFA levels during early seizures for 20:4, 22:6, and 18:0 was 3.6-, 2.5-, and 2.2-fold greater, respectively, when adjacent interictal activity was intense as compared to weak activity. During late seizures, this difference dropped to 2.2-fold for 20:4, the only FFA that showed a significantly higher value between robust versus weak interictal activity. In contrast, accumulation of DAG during early and late seizures was observed only when adjacent interictal activity was high. These results indicate that the cortical accumulation of FFA and DAG during ictal periods of similar intensity and duration depends upon the electrocortical activity during adjacent interictal periods.

Daily electroconvulsive shock treatment alters the inositol lipid system response in the rat hippocampus

The inositol lipid system (polyphosphoinositides and inositol phosphates) represents an important component of the cell signal transduction. Electroconvulsive shock (ECS) is known to activate cell signaling and lead to the release of second messengers. We tested the effects of daily ECS on the inositol lipid system and the generation of second messengers in vivo, by prelabeling the components of the system with [3H]myo inositol. The response to ECS was greater 30 sec after the sixth ECS, as compared to that obtained 30 sec after the first one. Also, rats killed 24h after the fifth ECS exhibited an increased PI labeling, as compared to rats handled for 6 days without receiving ECS. These results show that daily seizures (ECS-evoked) deeply modify the neuronal response to the stimulus, thus providing new information on the biochemical events involved in cell signal transduction during seizures.

The effect of a free radical scavenger and platelet-activating factor antagonist on FFA accumulation in post-ischemic canine brain

The effects of the platelet-activating factor antagonist BN 50739 and a free radical scavenger dimethyl sulfoxide on the accumulation of free fatty acids in post-ischemic canine brain are reported. Following 14 min of complete normothermic ischemia and 60 min of reperfusion, the total brain FFAs were approximately 150% higher than in the control group (p < 0.05). Perfusion with the platelet-activating factor antagonist BN50739 in its diluent dimethyl sulfoxide during 60 min of post-ischemic reoxygenation resulted in a 61.8% (p < 0.01) reduction in the total brain free fatty acid accumulation. Palmitic, stearic, oleic, linoleic, and arachidonic acids decreased by 53.8%, 63.5%, 69.0%, 47.4%, and 57.2%, respectively. Although dimethyl sulfoxide alone caused stearic and arachidonic acids to return to the normal concentration range, BN 50739 had a significant influence on recovery of palmitic, oleic, and linoleic acids and was previously shown to provide significant therapeutic protection against damage to brain mitochondria following an ischemic episode. Because free fatty acid accumulation is one of the early phenomena in cerebral ischemia, this study provides evidence to support the hypothesis that both platelet-activating factor and free radicals are involved in initiating cerebral ischemic injury.

A platelet-activating factor antagonist reduces corneal allograft inflammation and neovascularization

We assessed the role of platelet-activating factor (PAF) in corneal allograft rejection and evaluated the effects of a PAF antagonist on corneal inflammation, cellular infiltration, vascularization, and edema. Rabbits with vascularized corneas served as recipients of allogeneic cornea grafts. Rabbits with normal corneas underwent autografts as controls. All of the allografts developed the progression of signs characteristic of rejection. Nevertheless, treatment with the PAF antagonist BN52021 significantly inhibited corneal allograft vascularization for up to 10 days after transplantation and reduced the number of eosinophils in the allografts at 28 days after transplantation. In contrast, saline-treated allografts exhibited florid vascularization and intense inflammatory infiltrates. Control autografts survived without developing significant inflammation or vascularization. The retardation of allograft eosinophilia and graft vascularization by the PAF antagonist was most likely the result of suppression of PAF-mediated reactions in the cornea. These results indicate that PAF may play a role in corneal inflammation and vascularization after corneal transplantation, and that PAF antagonists may be clinically useful in delaying some of the pathophysiologic consequences of corneal graft rejection.

Platelet-activating factor as a potential retrograde messenger in CA1 hippocampal long-term potentiation

Long-term potentiation (LTP) refers to a persisting enhancement of neurotransmission that follows high-frequency activation of certain synapses. Although both pre- and postsynaptic mechanisms contribute to LTP, it is believed that the enhanced release of neurotransmitter that accompanies this process results from the production of a diffusible messenger in postsynaptic neurons which traverses the synaptic cleft and alters the function of presynaptic terminals. One candidate for such a messenger is arachidonic acid, a metabolite produced by phospholipase A2 which augments synaptic transmission when coupled with presynaptic stimulation. However, the effects of arachidonic acid require activation of the postsynaptic receptor for N-methyl-D-aspartate. Previously we found that platelet-activating factor (1 O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), another phospholipase A2-derived messenger, selectively enhances excitatory postsynaptic currents in hippocampal neurons by a presynaptic mechanism. We now present evidence that platelet-activating factor, acting at a receptor localized to synaptic regions, participates in LTP in the CA1 region of rat hippocampal slices and may serve as part of a retrograde signalling cascade.

Review: pharmacological manipulation of docosahexaenoic-phospholipid biosynthesis in photoreceptor cells: implications in retinal degeneration

Docosahexaenoic acid (22:6n-3, DHA) is derived in vertebrate animals from n-3 fatty acids present in the diet (i.e., alpha-linolenic acid, 18:3n-3 and/or other n-3-long chain polyunsaturated fatty acids) and is found in very high concentrations in phospholipids from membranes of the central nervous system. Disk membranes of photoreceptor outer segments and synaptic terminals display a preferential enrichment in DHA-phospholipids that appears to be necessary for normal excitable membrane functions. Because of the relevance of adequate DHA-phospholipid synthesis and sorting toward new assembled disk membranes and synaptic terminals, as well as the pathophysiological implications of abnormal DHA metabolism (including its synthesis, delivery to the retina, and incorporation into lipids by de novo and turnover pathways), we reviewed recent studies of: a) the preferential uptake and retention of DHA by photoreceptors and its metabolism as it is activated to DHA-CoA and incorporated preferentially into phospholipids; b) pharmacological manipulations using amphiphilic cationic drugs (i.e., propranolol) to show an active esterification of DHA into lipids via de novo synthesis; and c) perturbations in DHA metabolism in retinas from dogs with progressive rod-cone degeneration (prcd).

Differences in the acyl composition of the platelet-activating factor (PAF) precursor and other choline phosphoglycerides of the rabbit retinal rod outer segments and neural retina

Choline phosphoglycerides comprise almost half of vertebrate retinal phospholipids. This lipid pool contains the precursor of the potent lipid mediator, platelet-activating factor. The acyl composition and distribution of the different subclasses of the choline phosphoglycerides (alkylacyl-[or the precursor of platelet-activating factor], alkenylacyl-[or choline plasmalogen] and diacyl-glycero-3-phosphocholine) were studied in intact rabbit retina, neural retina and rod outer segments. Choline phosphoglycerides were isolated by high performance liquid chromatography and derivatized by acetylation after phospholipase C treatment. The derivatives were purified by high performance liquid chromatography and subjected to methanolysis. Fatty acids were analyzed by capillary gas liquid chromatography. In the intact retina and in the neural retina, the alkylacyl-glycero-3-phosphocholine and alkenylacyl-glycero-3-phosphocholine comprise 1.2% and 1.5%, respectively, of the total choline phosphoglycerides, whereas the rod outer segments contain twice the proportion of the precursor of platelet-activating factor and no detectable plasmalogens. On a mole percent basis, arachidonic acid was highest in the neural retinal alkenylacyl-glycero-3-phosphocholine (27%), 18% in the alkylacyl-glycero-3-phosphocholine and only 5% in the diacyl-glycero-3-phosphocholine. However, alkylacyl-glycero-3-phosphocholine from rod outer segments was enriched in docosapentaenoic acid (18%) while arachidonic acid was in the 3-4% range. Our results suggest that, in the neural retina, alkyl-arachidonoyl-glycero-3-phosphocholine is a source of both platelet-activating factor and of arachidonic acid which may be a substrate for both prostaglandins and lipoxygenase metabolites during an inflammatory episode and may contribute to the retinal pathology.

Platelet-activating factor is a messenger in the electroconvulsive shock-induced transcriptional activation of c-fos and zif-268 in hippocampus

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), undetectable in resting neural tissue, accumulates in brain during seizures. A hetrazepine, BN-50730, is shown here to displace [3H]PAF-specific binding from microsomal, but not from synaptosomal membranes, indicating selectivity for a high affinity intracellular binding site. Rats pretreated with BN-50730 by intraperitoneal or intracerebroventricular injection exhibited an inhibition of the electroconvulsive shock (ECS)-induced expression of c-fos and zif-268 in hippocampus. A much more pronounced, dose-dependent inhibition of ECS-induced zif-268 mRNA in hippocampus by intracerebroventricular injection of BN-50730 was observed. It is concluded that, in the hippocampus, PAF is a mediator of the expression of zif-268 and, to a lesser extent, c-fos through an intracellular specific binding site. Thus, PAF may be a messenger in signal regulated zinc-finger transcription factors, and in other immediate-early genes involved in long-term synaptic plasticity changes.

Docosahexaenoic acid is taken up by the inner segment of frog photoreceptors leading to an active synthesis of docosahexaenoyl-inositol lipids: similarities in metabolism in vivo and in vitro

Retinal uptake and metabolism of docosahexaenoic acid (DHA) was studied in vivo in frogs 1, 2, and 6 hours after dorsal lymph sac injections of [3H]-DHA (50 microCi/g). Light microscope autoradiography and biochemical techniques were used to compare the profiles of cellular uptake and lipid labeling with those obtained from 6 hour [3H]-DHA retinal incubations (final DHA concentration, 0.11 and 25 microM). Light microscope autoradiography demonstrated that rod photoreceptor ellipsoids and synaptic terminals preferentially labeled both in vivo and in vitro conditions. Also, the cytoplasm and oil droplets of retinal pigment epithelial cells became very heavily labeled after 6 hours of in vivo labeling. Phosphatidic acid showed the highest labeling in one hour, while other phospholipids accumulated label throughout the 6 hours. At that time point, most label was recovered in phosphatidyl-ethanolamine (37%), phosphatidylcholine (27%), and phosphatidylinositol (16%), the latter displaying 1.6-fold higher labeling than phosphatidylserine. The profile of labeled lipids was similar to that obtained in vitro when the concentration of DHA was in the nanomolar range. Our results suggest that de novo lipid synthesis is a major route for esterification of [3H]-DHA into retinal lipids, giving rise to an early and rapid labeling of DHA-phosphatidylinositol, both in vivo and in vitro, when DHA is present at low concentrations. Furthermore, the profile of labeled retinal cells under in vivo conditions closely resembles in vitro DHA labeling.

Coupling among energy failure, loss of ion homeostasis, and phospholipase A2 and C activation during ischemia

The objective of the present experiments was to correlate changes in cellular energy metabolism, dissipative ion fluxes, and lipolysis during the first 90 s of ischemia and, hence, to establish whether phospholipase A2 or phospholipase C is responsible for the early accumulation of phospholipid hydrolysis products. Ischemia was induced for 15-90 s in rats, extracellular K+ (K+e) was recorded, and neocortex was frozen in situ for measurements of labile tissue metabolites, free fatty acids, and diacylglycerides. Ischemia of 15- and 30-s duration gave rise to a decrease in phosphocreatine concentration and a decline in the ATP/free ADP ratio. Although these changes were accompanied by an activation of K+ conductances, there were no changes in free fatty acids until after 60 s, when free arachidonic acid accumulated. An increase in other free fatty acids and in total diacylglceride content did not occur until after anoxic depolarization. The results demonstrate that the early functional changes, such as activation of K+ conductances, are unrelated to changes in lipids or lipid mediators. They furthermore suggest that the initial lipolysis occurs via both phospholipase A2 and phospholipase C, which are activated when membrane depolarization leads to influx of calcium into cells.

Free fatty acid and diacylglycerol accumulation in the rat brain during recurrent seizures is related to cortical oxygenation

Cerebral blood flow and oxygenation increase during the early seizures of a series, but the increase in cerebral blood flow attenuates during late seizures, sometimes resulting in decreased cortical oxygenation. Cortical free fatty acids (FFA) and diacylglycerols also increase during early seizures and the increase attenuates during late seizures. We analyzed the correlation between lipid accumulation and cortical O2 during periodic pentylenetetrazol-induced seizures. During early seizures, both FFA and diacylglycerols increased in the cerebral cortex, particularly arachidonate (20:4) and stearate (18:0). Changes in lipids were different during late seizures, depending on cortical O2 levels. An increase in cortical O2 during late seizures was associated with lower FFA levels compared with early seizures, and FFA levels recovered to basal levels during interictal periods. A decline in cortical O2 was associated with a further increase in FFA, which remained elevated during interictal periods. Our results indicate that periseizure lipid accumulation is related to cortical oxygenation.

Decreased electroconvulsive shock-induced diacylglycerols and free fatty acid accumulation in the rat brain by Ginkgo biloba extract (EGb 761): selective effect in hippocampus as compared with cerebral cortex

The effect of Ginkgo biloba extract (EGb 761) treatment (100 mg/kg/day, per os, for 14 days) on electroconvulsive shock (ECS)-induced accumulation of free fatty acids (FFA) and diacylglycerols (DAG) was analyzed in rat cerebral cortex and hippocampus. EGb 761 reduced the FFA pool size by 33% and increased the DAG pool by 36% in the hippocampus. These endogenous lipids were unaffected in cerebral cortex. During the tonic seizure (10 s after ECS) the fast accumulation of FFA, mainly 20:4, was similar in sham- and EGb 761-treated rats, in both the cerebral cortex and hippocampus. However, further accumulation of free 18:0 and 20:4, observed in the hippocampus of sham-treated rats during clonic seizures (30 s to 2 min after ECS), did not occur in EGb 761-treated animals. The rise in DAG content triggered in the cortex and hippocampus by ECS was delayed by EGb 761 treatment from 10 s to 1 min, when values similar to those in sham animals were attained. Moreover, in the hippocampus the size of the total DAG pool was decreased by 19% during the tonic seizure. At later times, DAG content showed a faster decrease in EGb 761-treated rats. By 2 min levels of all DAG acyl groups decreased to values significantly lower than in sham animals in both cortex and hippocampus. This study shows that EGb 761 treatment affects, with high selectivity, lipid metabolism and lipid-derived second messenger release and removal in the hippocampus, while affecting to a lesser extent the cerebral cortex.

Platelet-activating factor induces collagenase expression in corneal epithelial cells

Platelet-activating factor (PAF), a potent lipid mediator involved in inflammatory and immune responses, accumulates rapidly in response to injury in a variety of tissues, including the corneal epithelium. However, the precise role of this compound in the cascade of events following insult has not been defined. Here we examined the effect of PAF on gene expression in the epithelial cells of rabbit corneas in organ culture. We found that incubation with 100 nM methylcarbamoyl PAF, a nonhydrolyzable analog of PAF, produced rapid transient 2.8- and 3.5-fold increases in the expression of c-fos and c-jun, respectively, at 1 hr, followed by increased expression of the collagenase type I gene beginning at 3 hr and peaking at 14-fold by 8 hr. Addition of the protein-synthesis-inhibitor cycloheximide superinduced c-fos and c-jun, strongly potentiating the PAF effect, but inhibited the induction of collagenase type I expression, suggesting the existence of a transcriptional factor linking the two events. BN-50730, a selective antagonist of intracellular PAF-binding sites, blocked the expression of the immediate-early genes as well as the increase in collagenase type I mRNA. Our results suggest that one of the functions of PAF may be to enhance the breakdown of the extracellular matrix as a part of the remodeling process during corneal wound healing after injury. Pathologically, a PAF-induced overproduction of collagenase may be a factor in the development of corneal ulcers, as well as other pathophysiological conditions such as cartilage destruction in arthritis. If so, inhibitors of this signal-transduction pathway may be useful as tools for further investigation and, eventually, as therapeutic agents to treat such disorders.

Pathways for the uptake and conservation of docosahexaenoic acid in photoreceptors and synapses: biochemical and autoradiographic studies

Docosahexaenoic acid (22:6n-3) esterified into phospholipids represents by far the most prevalent fatty acid of rod photoreceptor disc membranes and synaptic terminals. During synaptogenesis and photoreceptor biogenesis, plasma lipoproteins, secreted mainly by the liver, are the main source of plasma 22:6n-3 for the central nervous system. This systemic route (the long loop) also operates in mature animals for morphogenesis and maintenance of excitable membranes (e.g., during constant renewal of photoreceptor disc membranes). When radiolabeled 18:3n-3, the dietary precursor of 22:6n-3, is systemically supplied to 3-day-old mouse pups, it is elongated and desaturated in the liver, leading to the synthesis of 22:6n-3-lipoproteins that shuttle the fatty acid through the bloodstream to retina and brain. When radiolabeled 22:6n-3 was used, a more efficient labeling of brain and retinal lipids was achieved. The retinal pigment epithelium is involved, not only in the uptake of 22:6n-3 from circulating lipoproteins in the choriocapillaris but also in the recycling of 22:6n-3 from degraded phagosomal phospholipids back to the inner segments of photoreceptors (the short loop), following each phagocytic event. An interplay among efficient 22:6n-6 delivery from the liver, selective uptake by retinal pigment epithelium photoreceptor cells, and avid retinal retention may contribute to the enrichment of excitable membranes of the retina with 22:6n-3-phospholipids.

Visualization of [3H]docosahexaenoic acid trafficking through photoreceptors and retinal pigment epithelium by electron microscopic autoradiography

PURPOSE

[3H]docosahexaenoic (DHA) acid was followed through the retinal pigment epithelial cells and photoreceptors for up to 5 days after injection to specifically determine which membrane systems of the retinal pigment epithelial cells are used in the handling of [3H]DHA after shedding and phagocytosis of rod tips.

METHODS

Frogs (Rana pipiens) were injected with [3H]DHA in the dorsal lymph sacs, and maintained for up to 5 days. Retinas were processed for electron microscopic autoradiography, stored for various periods of time, and then analyzed by transmission electron microscopy.

RESULTS

After 1 day, [3H]DHA had accumulated within photoreceptor ellipsoids, and had begun to appear as dense label in newly formed discs. By day 5, the basal region of dense label had expanded apically. Newly shed rod outer segment tips were diffusely labeled; but occasionally after several hours, they acquired additional label as they moved near Bruch's membrane. Retinal pigment epithelial cytoplasm maintained a constant level of label, with myeloid bodies sometimes slightly labeled. Oil droplets of the retinal pigment epithelium accumulated dense label throughout this study.

CONCLUSIONS

When [3H]DHA enters the retinal pigment epithelium, some is retained within oil droplets, whereas the rest is passed on to the photoreceptors. [3H]DHA is initially taken up by inner segments and then dispersed to photoreceptor synaptic terminals as well as to ellipsoids where discs are assembled. Phagosomal labeling exactly matches rod outer segment tips, but occasionally increases as degradation occurs near Bruch's membrane. Normally, density of label remains constant throughout the degradation process.

EGb 761 inhibits stress-induced polydipsia in rats

The effect of daily treatments with Ginkgo biloba extract (EGb 761, IPSEN, France) on body weight and water intake of rats was followed for 15 days. During this period, two groups of rats, under slight ether anesthesia, were intubated and fed either EGb 761 (100 mg/kg b.wt. in 5% ethanol) or, for sham controls, 5% ethanol alone (6.6 ml/kg b.wt.). The increase in body weight was similar for the control and experimental groups. However, during the same period of time, the water intake, ml water/g b.wt./24 h, increased 37% in the controls. In EGb 761-treated rats, water intake remained unchanged. This suggests that EGb 761 treatment inhibits the development of polydipsia due to the stress of daily handling and intubation.

The activation of phospholipase A2 and release of arachidonic acid and other lipid mediators at the synapse: the role of platelet-activating factor

Seizures promote PLA2 activation which is selectively detectable in isolated synaptosomes by an increased free arachidonic acid (AA) and docosahexaenoic acid (DHA) pool size. During long-term potentiation, a role of AA and its oxygenated metabolites has been explored in several laboratories. We have studied another PLA2 product, platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-glycerophosphocholine) that is also generated during intense synaptic activity such as seizures. We found specific PAF binding sites in both synaptic and intracellular membranes. Using rat postnatal hippocampal synaptic pairs we have shown that PAF specifically increases the release of excitatory neurotransmitter. This effect is elicited through the synaptic binding site since an antagonist selective for this site blocks the PAF-mediated increase in excitatory neurotransmitter release. Although PAF augments evoked excitatory synaptic currents, it does not alter GABA-mediated inhibitory currents. PAF increases the frequency but not the amplitude of spontaneous excitatory synaptic minis. At present it is not known if the phospholipase A2 that accumulates free polyunsaturated fatty acids is the same as the one that gives rise to PAF. This lipid mediator effect on excitatory synaptic transmission may be a critical step in long term potentiation, synaptic plasticity, memory formation and epileptogenesis.

Electroconvulsive shock alters GABAA receptor subunit mRNAs: use of quantitative PCR methodology

Electroconvulsive shock (ECS) may affect several neurotransmitter systems in brain, including the GABAergic inhibitory system. We used a quantitative PCR-based assay to evaluate mRNAs for five GABAa receptor subunits at 2 to 24 h after ECS. mRNAs for the alpha 1 and beta 2 subunits were significantly increased in cerebellum at 4 and 8 h after ECS, and returned to control levels at 24 h. No changes were observed in alpha 2, beta 3, gamma 1, or gamma 2 subunits, and no changes in any subunit evaluated were observed in cortex or hippocampus. These data corroborate prior results obtained for the alpha 1 subunit using Northern hybridization, and illustrate the utility of the PCR assay in quantitating low-abundance mRNAs.

Excitable membranes, lipid messengers, and immediate-early genes. Alteration of signal transduction in neuromodulation and neurotrauma

The physical nature of neuronal cells, particularly in the functional and morphological segregation of synapse, soma, and dendrites, imparts special importance on the integrity of their cell membranes for the localization of function, generation of intrinsic second messengers, and plasticity required for adaptation and repair. The component phospholipids of neural membranes are important sources of bioactive mediators that participate in such diverse phenomena as memory formation and cellular damage following trauma. A common role for PAF in these processes is established through the suppressive effects of its antagonists. Furthermore, being both an extracellular and intracellular agonist of phospholipase activation, in addition to being a product of phospholipase activity, PAF assumes a centralized role in the cellular metabolism following neural stimulation. The linkage of PAF to neural immediate-early gene expression, both in vitro and in vivo, suggests that its effects are initiating to long-term formative and reparative processes. Such a common link between destructive and plastic responses provides an important view of cellular and tissue maintenance in the nervous system.

Enhancement of hippocampal excitatory synaptic transmission by platelet-activating factor

The biologically active lipid platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine; PAF) is a mediator of inflammatory and immune responses, and it accumulates in the brain during convulsions or ischemia. We have examined whether PAF may play a second messenger role in the central nervous system by studying effects on synaptic transmission in cultured hippocampal neurons. Carbamyl-PAF, a nonhydrolyzable PAF analog with a similar pharmacologic profile, augmented glutamate-mediated, evoked excitatory synaptic transmission and increased the frequency of spontaneous miniature excitatory synaptic events without increasing their amplitude or altering their time course. This compound had no significant effect on gamma-aminobutyric acid-mediated inhibitory synaptic responses. Lyso-PAF, the biologically inactive metabolic intermediate, had no effect on synaptic transmission. Moreover, the enhancement of excitatory synaptic transmission by carbamyl-PAF was blocked by a PAF receptor antagonist. These results indicate a specific presynaptic effect of PAF in enhancing excitatory synaptic transmission in cultured rat hippocampal neurons.

Light stimulates in vivo inositol lipid turnover in frog retinal pigment epithelial cells at the onset of shedding and phagocytosis of photoreceptor membranes

We have developed an experimental model to study in vivo inositol lipid metabolism in frog retinal pigment epithelial (RPE) cells, including the effect of light on phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. RPE cells were rapidly isolated after either brief light or dark periods. Light and electron microscopy showed complete detachment of the retina from the RPE cells, and that the RPE cell suspensions were devoid of photoreceptor cell outer segments. Frog tissues were labeled in vivo for 20 hr by intravitreal injection of [3H]inositol (4 microCi, 4 microliters per eye) within a 24-hr constant illumination period. Following 1 hr of darkness (priming period), frogs were intravitreally injected with LiCl (0.5 M, 4 microliters per eye) 15 min before the onset of either 30-min light stimulation or an additional 30 min of darkness (controls). In order to preserve endogenous inositol phosphate pools present after dark and light exposure, the RPE cells were harvested in the shortest time possible, at low temperatures (18-20 degrees C), and in the presence of 10 mM LiCl. Total [3H]inositol-labeled water-soluble products (inositol plus inositol phosphates) were increased by 86% after 30 min of light. Inositol trisphosphate (IP3) showed the highest accumulation (a 5.5-fold increase), followed by inositol bisphosphate (1.9-fold increase) and inositol monophosphate (1.4-fold increase). Free [3H]inositol also accumulated (2.8-fold increase), reflecting only a partial inhibition of phosphomonoesterase by LiCl. These changes were paralleled by a 12% decrease in 3H-labeled phosphatidylinositol with no significant difference in the labeling of polyphosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)

Generation of arachidonic acid and diacylglycerol second messengers from polyphosphoinositides in ischemic fetal brain

Intracerebral administration of [3H]arachidonic acid ([3H]ArA) into 19-20-day-old rat embryos, resulted in a rapid incorporation of label into brain lipids. One hour after injection, 55.6 +/- 8.2, 18.0 +/- 3.4, and 13.7 +/- 1.3% of the total radioactivity was associated with phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, respectively. Approximately 10% of radioactivity was found acylated in neutral lipids of which free ArA comprised only 1.5 +/- 0.2% of the total radioactivity. Complete restriction of the maternal-fetal circulation for < or = 40 min did not affect the rate of [3H]ArA incorporation (t1/2 = 2 min) into fetal brain lipids, suggesting an effective acylation mechanism that proceeds irrespective of the impaired blood flow. After a short restriction period (5 min), the radioactivity in diacylglycerol was elevated by 50%. After a longer restriction period (20 min), the radioactivity in the free fatty acid and diacylglycerol fractions increased to values of 130 and 87%, respectively. Polyphosphoinositides prelabeled with either [3H]ArA or 32P were rapidly degraded after 5 min of ischemia. After 20 min, the decrease in phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate radioactivity was 47 and 70%, respectively. Double labeling of phospholipids with [14C]palmitic acid and [3H]ArA indicated a preferential loss of [3H]ArA within the polyphosphoinositide species after 20 min, but not after 5 min of ischemia. The specific activity of [14C]palmitate remained unchanged. The current data suggest phospholipase C-mediated diacylglycerol formation at the beginning of the insult followed by a phospholipase A2-mediated ArA liberation at a later time, both enzymes presumably acting preferentially on polyphosphoinositide species.

Growth-associated protein GAP-43 and nerve cell adhesion molecule in sensory nerves of cornea

We used dual-wavelength fluorescence microscopy and monoclonal antibodies to growth-associated protein (GAP-43) and nerve cell adhesion molecule (N-CAM) to identify these proteins in nerve fibers of normal rat and rabbit corneas. Overlapping immunoreactivity of GAP-43 and N-CAM was evident along nerve fibers of rabbit corneal sections, suggesting that GAP-43 is constitutively expressed in these sensory nerves. The immune reaction of monoclonal antibody to GAP-43 and [125I]protein A was used to quantitate relative amounts of GAP-43 in the normal cornea and in a cornea subjected to a de-epithelializing wound. Collectively these findings imply that GAP-43 is axoplasmically transported from cells in the trigeminal (or superior cervical) ganglion to the cornea. Moreover, these data indicate that GAP-43 appears to be involved in the remodeling of corneal nerves that is necessary for normal innervation.